In recent years, a micro-wave electron tube such as a klystron or the like have had a tendency to exhibit a high output. Particularly, those tubes which are used in a plasma apparatus for nuclear fusion or a particle accelerator exhibit an output of a megawatt or more. A much higher output is required for those tubes. Meanwhile, there have been demands for developments in a color picture tube improved in resolution by increasing scanning lines and a super high frequency responsive picture tube, and hence, improvements in brightness have been required. Improvements in brightness have also been required for a projection tube. To respond to these requirements and demands, the emission current density of a current from a cathode must be greatly increased in comparison with a conventional apparatus.
Several conventional electronic tubes such as a color picture tube used in a color picture receiver require a high voltage supplied to a convergence electrode, a focus electrode or the like, in addition to an anode voltage. In this case, a problem issues in the aspect of a withstand voltage if a high voltage is supplied from a stem portion of the color picture tube. Therefore, a method is adopted in which a resister for a divisional voltage together with an electron gun are incorporated as a electron-gun built-in resister into the color picture tube and in which an anode voltage is divided to supply high voltages to electrodes, respectively.
Starting from studies made in 1939, developments have been made to use this tube as an amplifier tube, an oscillation tube, or the like which can widely response to an UHF band to a milli wave range. In 1960s, further developments have been started to use a klystron tube for a satellite communication earth station. In 1970s, studies have been promoted in view of high efficiency operation of a klystron tube, and products with an efficiency of 50% or more have been put into practical use including UHF-TV broadcasting. Recently, a klystron tube of a super high power has been developed which attains an efficiency of 50 to 70%, a continuous wave output of 1 MW, and a pulse output of 150 MW, and has been used in an accelerator of a super large scale, a plasma heating apparatus for nuclear fusion studies. A klystron tube can generate a high power at a high efficiency, and is therefore used widely in the field of high power tubes.
A traveling wave tube was invented in 1943 and was completed thereafter. There are various types of traveling wave tubes, such as a spiral type, a cavity coupling type, a cross finger type, a ladder type, and the likes. A traveling wave tube of a spiral type has been widely used as a transmitting tube to be mounted on an air-plane, an artificial satellite or the like. A cavity connection type traveling wave tube has been developed for the purpose of compensating for a withstanding power capacitance of a spiral type, and has been put into practice mainly as a transmitting tube for a satellite communication earth station. Although a traveling wave tube normally attains an efficiency of about several to 20%, a traveling wave tube which attains an efficiency of 50% has been developed for a satellite when electrical potential depression-type corrector is provided with the traveling wave tube.
Meanwhile, as well-known, a gyrotron tube is an electron tube based on an operation principle of a cyclone maser effect, and is used as a high frequency high power source which generates a high power milli wave of several tens to several hundreds GHz.
An impregnated-type cathode ensures a higher emission current density than an oxide cathode, and has therefore been used as an electron tube for a cathode ray tube, a traveling wave tube, a klystron tube, a gyrotron tube, or the like. Use of an impregnated-type cathode has been limited to particular applications such as an HD-TV tube, an ED-TV tube, and the likes, in the field of color picture tubes. However,.demands for a large-size CRT and the likes have increased in recent years, and the use filed of an impregnated-type cathode has been rapidly expanded.
For example, in case of an impregnated-type cathode assembly used in klystron tubes and color picture tubes, the cathode substrate is made of porous tungsten (W) of a porosity 15 to 20%, and the porous portion of this cathode substrate is impregnated with electron emission substances such as barium oxide (BaO), calcium oxide (CaO), aluminum oxide (Al.sub.2 O.sub.3), and the likes. Further, an iridium (Ir) thin film layer is provided on the electron emission surface of the cathode substrate by a thin film formation means like a sputtering method, thereby using an impregnated-type cathode assembly coated with iridium.
In this cathode assembly, for example, barium (Ba) and oxygen (O.sub.2) impregnated in the cathode assembly is diffused by an aging step after the cathode assembly is mounted in the electron tube, so that dipole layer is formed on the electron emission surface of the cathode assembly surface. As a result, a high emission current is enabled.
Although the aging time in an aging step is variously arranged in accordance with an applied voltage during use of an electron tube as a target, an dipole layer can be formed in an aging time of about 50 hours in case of an electron tube used in low voltage operation, for example, with an applied voltage of about 10 kV.
On the contrary, in case of an electron tube used in high voltage operation, e.g., a super high power klystron tube used with an applied voltage of 70 kV, a current of a sufficient current density can be picked up by aging of a relatively short time period of several tens hours where a current picked up has a pulse width of 5 .mu.s and is repeated for 500 times for every one second. However, if a current thus picked up is a direct current, aging requires 500 hours or more to pick up a current of an equal current density.
In case of an electron tube such as a super high power klystron tube used in high voltage operation, a large amount of gas emitted from a collector is collided with electrons to be ionized at the same time when an dipole layer is formed by means of aging. Further, these ions collide with an electron emission surface due to a high voltage, thereby breaking the dipole layer. In this state, the ionized gas has a high energy. As the amount of gas which collides with the electron emission surface increases, the dipole layer of the electron emission surface is broken seriously. Therefore, an electron tube used in high voltage operation requires aging of a long time.
In addition, an impregnated-type cathode assembly for a cathode ray tube is formed to have a compact structure for the purpose of energy saving. Therefore, an impregnated-type cathode assembly for a cathode ray tube has a limited thickness and a limited diameter which make it difficult to impregnate a sufficient amount of electron emission substance. Generally, the characteristics of the life-time of an impregnated-type cathode are dependent on the amount of evaporation of barium as a main component of electron emission substance. As barium is consumed by evaporation, the monolayer covering late decreases. Electron emission ability decreases in accordance with an increase in the work function. As a result of this, the long life-time characteristic cannot be achieved. This is a large practical problem. From this stand of view, an impregnated-type cathode assembly is desired which can be operated at a low temperature.
In recent years, attentions have been paid to a scandium-based (or Sc-based) impregnated-type cathode assembly as such a cathode assembly for a cathode ray tube.
The scandium-based impregnated-type cathode assembly described above has an excellent pulse emission characteristic at a low duty, in comparison with an impregnated-type cathode assembly coated with metal, and is expected to be capable of operating at a low temperature.
However, in this scandium-based impregnated-type cathode assembly which can be operated at a low temperature, recovery of lost Sc is slow and the operation ability at a low temperature is lowered if the cathode once receives an ion impact under a condition of a high frequency. Thus, this assembly is not sufficiently practicable.
For example, in case of a type in which a scandium compound is covered over the surface of the cathode substrate, the surface state changes during steps of manufacturing a cathode. Operation over a long time leads to dissipation of scandium and to deterioration in the electron emission characteristic. In addition, the surface of the substrate is locally broken due to ion impacts, and the work function of broken portions is raised so that the distribution of electron emission becomes non-uniform.
As a result of Auger surface analysis in a scandium-based impregnated-type cathode, it has been determined that scandium on the surface is lost upon an ion impact and recovery-of an excellent density of electron emission requires a long time, in case of a scandium-based impregnated-type cathode.
The followings are examples of a conventional cathode substrate.
Japanese Patent Application KOKAI Publication No. 56-52835 and Japanese Patent Application KOKAI Publication No. 58-133739 disclose a cathode substrate in which a cover layer having a porosity of 17 to 30% is provided on a porous substrate, and this porosity of the cover layer is lower than that of the porous substrate. However, in this kind of cathode substrate, the porosity of the cover layer is arranged to be low, and therefore, evaporation of an electron emission substance is restricted to be low, so that the life-time of the cathode can be elongated. However, under operating condition that ion impacts are strong as in an electron tube which operates at a high current density, recovery of the structure of the cathode substrate surface is late, so that excellent results cannot be obtained. Japanese Patent Application KOKAI Publication 58-177484 discloses a cathode substrate containing scandium, which cannot attain sufficient recovery of scandium after an ion impact. Therefore, this cathode substrate achieves only an insufficient low-temperature operation ability. Japanese Patent Application KOKAI Publication 59-79934 discloses a cathode substrate in which a layer containing high melting point metal and scandium is formed on a high melting point metal layer. In this cathode substrate, recovery of scandium after an ion impact is not sufficient, and therefore, a sufficient operation ability at a low temperature cannot be attained.
Japanese Patent Application KOKAI Publication 59-203343 discloses a cathode substrate in which a uniform layer containing fine tungsten of 0.1 to 2 .mu.m, scandium oxide and electron emission substances is formed on a porous base made of tungsten. This cathode substrate contains scandium, and therefore, can be operated at a low temperature. However, under operating condition that ion impacts are strong, recovery of the structure of the cathode substrate surface is late, so that excellent results cannot be obtained. Japanese Patent Application KOKAI Publication 61-91821 discloses a cathode substrate in which a cover layer made of tungsten and scandium oxide is provided on a porous substrate. This cathode substrate contains scandium, and therefore, can be operated at a low temperature. However, under operating condition that ion impacts are strong, recovery of the structure of the cathode substrate surface is late, so that excellent results cannot be obtained. Japanese Patent Application KOKAI Publication 64-21843 discloses a cathode substrate in which a first formed body having a large average particle diameter of, for example, 20 to 15 .mu.m is provided, and a top head whose average particle diameter is smaller than that of the first formed body is provided on the first formed body. In this cathode substrate, evaporation of an electron emission substance is restricted to be low, and therefore, the life-time of the cathode can be elongated. However, under operating condition that ion impacts are strong, recovery of the structure of the cathode substrate surface is late, so that excellent results cannot be obtained.
Further, Japanese Patent Application KOKAI Publication 1-161638 discloses a cathode substrate in which a layer of scandium compound or scandium alloy is provided on a porous substrate made of high melting point metal. Japanese Patent Application KOKAI Publication No. 3-105827 and Japanese Patent Application KOKAI Publication No. 3-25824 disclose a cathode substrate in which a layer of a layered structure or of a mixture substance is formed on a porous substrate. The layered structure consists of a mixture layer of tungsten and scandium oxide, and a layer of a scandium supplier, e.g., Sc combined with Re, Ni, Os, Ru, Pt, W, Ta, Mo, or the like. The mixture substance is made of these materials. Japanese Patent Application KOKAI Publication No. 3-173034 discloses a cathode substrate in which a layer containing barium and scandium is included as an upper layer of a high melting point metal porous substrate. Japanese Patent Application KOKAI Publication No. 5-266786 discloses a cathode substrate in which, for example, a layered structure containing high melting point metal such as a tungsten layer, a scandium layer, a rhenium layer and the like is formed on a porous substrate made of high melting point metal. However, the cathode substrates described above cannot ensure sufficient recovery of scandium after an ion impact, the low-temperature operation ability is insufficient. Thus, a sufficient ion-impact resistance cannot be attained.